Lescoop77 has great advice, but I would do it slower. Do one action for a few days at a time before moving onto the next step. And don't stop at the one block marker, continue it to a few blocks, then a mile then a few miles.
If for any reason he improves then starts to regress you should go back a step (or more if needed) until he feels comfortable and does not get sick, then start to move forward again.

Also by taking him on "fun" rides will help too. Like to the park, friends houses etc... instead of always the vets & such.

And you can check with your vet for the doseage but you can give him dramamine (sp?) to help with motion sickness.

Good luck!